The present invention concerns data slicers or quantizers and in particular, a data slicer suitable for use in a baseband or passband decision-feedback equalizer.
Equalizers are typically used in coded digital communications systems to compensate for multipath/linear filtering effects caused by the transmission channel. These effects are commonly referred to as channel impairments and include signal distortion that may occur in the transmitter, in the receiver or in the channel through which the signal is transmitted. The equalizer is an adaptive filter, often implemented as a finite impulse response (FIR) filter, an infinite impulse response (IIR) filter or a combination of FIR and IIR filters. Each filter has a plurality of coefficients that are adapted to minimize an error criterion. This error criterion may be, for example, the mean-square error between a transmitted training signal and the received training signal. A typical equalizer maintains a copy of the transmitted training signal to compare with the received training signal. It is generally believed that a decision-feedback equalizer (DFE) has better asymptotic performance than a linear equalizer as described in a text book by J. G. Proakis, entitled Digital Communications. 
A typical DFE is shown in FIG. 1. The received signal is applied to an FIR filter and the output signal produced by the FIR filter is applied to an IIR filter. The IIR filter includes a subtracter 111, a slicer 112 and an IIR filter section 114. The subtracter 111 subtracts the filtered signal provided by the IIR filter section 114 from the output signal of the FIR filter 110. The slicer 112 quantizes the signal provided by the subtracter 111 to produce an approximation of the signal that was transmitted. The IIR filter section, which may be, for example, an FIR filter in a feedback loop, processes the quantized signal to produce the signal that is subtracted by the subtracter 111. For an uncoded modulation scheme, the DFE uses the slicer to get decisions for the feedback portion. The slicer is a nearest element decision device which returns the source constellation member closest to its input. In decision directed operation, the output signal of the slicer is compared to its input signal to determine in what way the coefficients of the FIR and IIR filter sections should be updated to minimize any differences between the signal that was recovered and the known training signal.
For a coded modulation scheme, it may be desirable to replace the slicer with a decoder, that may include, for example, a trellis decoder, a deinterleaver, and a Reed Solomon (RS) decoder. The use of such a decoder, however, delays the decision on what symbol was transmitted by several symbol periods. These delays can be prohibitive for the DFE, since it relies on canceling the inter-symbol-interference of the previous symbols on the current symbol by using previously available decisions. Hence, the state of the art has typically not used a complete decoder, but a range of simplified decoders including the simple slicer 112, that does not perform any decoding. A typical problem with using only a slicer in a DFE is a loss in performance due to incorrect decisions. Because an incorrect decision used in the DFE to remove inter-symbol interference (ISI) can cause further errors, this performance loss is known as “error propagation.”
More complex decoding techniques may also be used, for example, Reduced-State Sequence Estimation (RSSE) and parallel decision feedback decoding (PDFD). These techniques are described in an article by V. Eyuboglu and S. Qureshi, entitled “Reduced-State Sequence Estimation for Coded Modulation on Intersymbol Interference Channels” IEEE Journal on Selected Areas of Communications, August 1989. Furthermore, U.S. Pat. No. 5,056,117 entitled DECISION-FEEDBACK EQUALIZATION WITH TRELLIS CODING to R. Gitlin, describes a method by which multiple possible decisions are fed back and the best among them is chosen using a given criteria. Other techniques are described in an article by A. Duel-Hallen and C. Heegard, entitled “Delayed Decision-Feedback Equalization”, IEEE Transactions on Communications May 1989. All of the above cited references are incorporated herein by reference for their teachings on combined equalization and decoding techniques.
Generally the common idea among these decoders is to use multiple possible decisions or to use a more complicated trellis decoder that includes a channel state estimate. The implementation complexity of these approaches, however, is significant and may undesirably add to the cost of the decoder.
U.S. Pat. No. 5,923,711 entitled SLICE PREDICTOR FOR A SIGNAL RECEIVER to Willming describes a slicer for a trellis coded vestigial sideband (VSB) signal that estimates a current symbol using a partial estimate of the current symbol which is derived from the previous symbol. The partial estimate is derived only from the previous symbol and is used to reduce the probability of error in the estimate of the current symbol. The system disclosed by Willming recovers carrier frequency from the pilot signal of the VSB signal. The pilot signal of a VSB signal, is not a good reference, however, as it may be corrupted both in frequency and phase by multipath distortion. In addition, the system disclosed by Willming may not operate properly when used with VSB signals that do not have pilot signals.